1. Field of the Invention
The present invention relates to a method for manufacturing a bipolar transistor.
2. Description of the Related Art
In a first prior art method for manufacturing a bipolar transistor, a first insulating layer, a first polycrystalline silicon layer of a second conductivity type and a second insulating layer are sequentially formed on a semiconductor substrate of a first conductivity type. Then, the second insulating layer and the first polycrystalline silicon layer are patterned to form an opening therein. Then, the first insulating layer is overetched by using the second insulating layer and the first polycrystalline silicon layer as a mask. Then, a second polycrystalline silicon layer is formed on the entire surface. Then, the second polycrystalline silicon layer except for a part of the second polycrystalline silicon layer under the first polycrystalline silicon layer is etched by an isotropic etching process. Then, impurities of the second conductivity type are implanted into the semiconductor substrate to form a base region. Then, a sidewall insulating layer is formed on a sidewall of the first and second polycrystalline silicon layers. Then, a third polycrystalline silicon layer of the first conductivity type is formed on the base region. Finally, an annealing operation is carried out. As a result, impurities of the second conductivity type are diffused from the first polycrystalline silicon layer via the second polycrystalline silicon layer into the semiconductor substrate to form a graft base region, and impurities of the first conductivity type are diffused from the third polycrystalline silicon layer into the base region to form an emitter region. This will be explained later in detail.
In the above-described first prior art method, however, since the semiconductor substrate is also etched at the isotropic etching process for etching the second polycrystalline silicon layer, the transistor characteristics are deteriorated. For example, a leakage current may be increased, and also the graft base region cannot be electrically connected to the base layer which increases the base resistance.
In a second prior art method for manufacturing a bipolar transistor, the second polycrystalline silicon layer of the first prior art method is oxidized, and the oxidized part thereof is removed by a dry etching process (see JP-A-62-169364).
In the above-described second prior art method, however, since the semiconductor substrate is damaged by the dry etching process for removing the oxidized part of the second polycrystalline silicon layer, the transistor characteristics are still deteriorated.